On the Genesis of Some Scottish Minerals. 215 



the introduction of some constituent from an outside source, 

 such as alkalies, from sea-water. 



Another, and very important, effect of this kind of hydro- 

 metam Orphic action manifests itself in connection with 

 volcanic rocks. As a volcano increases in size (of course, 

 mainly by additions to its exterior), the lines of equal 

 internal temperature (the volcanic isogeotherms) keep pace 

 with its growth, and probably remain at nearly the same 

 depth below the surface of the cone throughout all stages in 

 the history of the volcano. It follows from this that the 

 temperature of the HoO permeating the central part of a 

 volcano increases with the growth of the cone. It ensues 

 as a natural result that a gradual re-softening of the earlier 

 deposited beds of lava, tuff, and intrusive masses, may 

 commence soon after they have been deposited. In those 

 cases in which this stage is reached before the volcanic rocks 

 have lost any of their alkalies, a molecular reconstruction 

 of the constituents must necessarily arise. In this process, 

 there can be no doubt that many fragments of crystals which 

 were ejected during explosive eruption have been " mended," 

 and others have increased in size by additions to their 

 exterior. Furthermore, as tuffs usually consist of little else 

 than fragments of what would have passed into rocks of 

 holocrystalline structure had the fluid mass remained long 

 enough undisturbed by explosive forces, a comparatively 

 slight change suffices to convert these fragments into rock 

 undistinouishable from lava, or even, in extreme cases, from 

 one or other of the plutonic representatives of such effusive 

 rocks. The importance of the fact in question cannot be 

 doubted. A recent visit (June 1899) to the Ordovician 

 rocks of the English Lake District has convinced me that a 

 large part of the volcanic rocks which are regarded by many 

 petrographers as lavas, are really, as Mr Aveline and the 

 late Mr J. C. Ward considered them, simply pyroclastic rocks 

 thus altered contemporaneously. 



Mention has more than once been made above to the fact 

 that some of the andesitic tuffs which have been thermo- 

 metamorphosed next the diorite of Tillicoultry, in the 



